A common task while testing UIMA components is (or at least should be) validating that the annotations that have been created by some annotator or reader actually conform to the expectations you have for your type system. E.g. stuff like annotations not ending before they start (which is actually a general requirement for UIMA text-oriented annotations).
So it would be good if type systems should ship with a bunch of validator classes that some central validator code could auto-discover. The validator code would then be called e.g. in unit tests in order to check that the output of a component is valid with respect to the intentions of the type system.